Method of forming patterned metalization on patterned semiconductor wafers

ABSTRACT

A metalization process forms metal contacts having defined profiles for contact between microelectromechanical (MEMS) devices or chemical sensors with semiconductor devices. Gold contacts may be used for connecting the MEMS devices or chemical sensors to integrated CMOS devices. Gold contacts are deposited over a photoresist via having sidewalls for forming upwardly extending flanges. The metal contacts to the underlying semiconductor device, are formed using a polymethylmethacrylate (PMMA) etch back process for exposing and dissolving the gold metalization layer save the metal contact under a surviving portion of the etched back PMMA layer in a dimple of the gold layer over the photoresist via. The photoresist layer serves to form deep well gold contacts having upwardly extending flanges for connection to the MEMS devices or chemical sensors and to the integrated semiconductor devices.

FIELD OF THE INVENTION

The invention relates to the field of semiconductor processing. Moreparticularly the present invention relates to etch back methods forforming sensor contacts during thin film semiconductor processing.

BACKGROUND OF THE INVENTION

The microelectromechanical systems (MEMS) are being manufactured usingprocess steps often found in traditional semiconductor processes. MEMSfabrication services are becoming widely used in many desirabledevelopmental variations. The use of MEMS technology often presentsdifficult challenges when integrating MEMS devices into and withcompatible semiconductor devices and processes. The semiconductorprocesses cover many types of devices and materials. One suchsemiconductor device and process is complementary metal oxide silicon(CMOS) technology. The CMOS process has been traditionally used forfabricating fast low power digital devices. Most MEMS devices are analogtype devices. Complete system designs often require the speed andaccuracy of modern digital computer processing systems that are coupledto the real world using analog input and output devices. Complete systemdesigns lead to the integration of digital devices and analog devices ona chip with the advantage of an economy of scale. However, suchintegration of different devices and the corresponding different processsteps must be accomplished with inherent compatibility. Many analogdevices operate using gold connector contacts because gold is a goodelectrical conductor that is also non-corrosive and durable. Aluminum isa good conductor, but is highly corrosive, and not desirable for use asan exposed conducting contact. Gold is a large atom, and gold atomicmigration through the lattice structures of semiconductor devices oftenleads to a decrease in the mean time between failure as gold atomsfunction as an impurity when migrating from an original deposition site.Though highly conductive, gold and silver impurities near the gatejunctions of metal oxide silicon (MOS) transistors can lead to prematurefailures. Hence, in CMOS semiconductor circuits, often polysilicon,aluminum, and tungsten are used as conductors to avoid the migrationproblem when using gold or silver.

A sensor contact metal, such as gold or tungsten, can be deposited in acontact via well leading to a semiconductor device in a preexistingsemiconductor circuit. For example, a sensor contact metal can bedeposited over the contact via well with a potentially corrosive analogsensor then being deposited onto the sensor contact metal. The sensorcontact metal can then be covered by a deposited protection layer toprotect the sensor and contact metal from corrosion when the sensor isexposed to the environment. As a preexisting example, a siliconsubstrate may have an aluminum conducting etch run that is covered by aninsulation layer such as silicon dioxide. During photoresistapplication, mask exposure and development, a contact via is formedthrough the photoresist. Photoresist is usually applied by spinning acoating onto a silicon wafer. The silicon dioxide layer is then etchedin the location of the photoresist via to form the contact via throughthe silicon dioxide layer. The photoresist layer is then removed leavingthe silicon dioxide layer over the aluminum conductor excepting for thecontact via through the silicon dioxide layer. The formation of thecontact via through the silicon dioxide layer to the conductor etch runof the semiconductor is an initial starting process point for depositingsensor contact metal upon a buried conductor etch run, prior to thendepositing the sensor on the contact metal. The metal sensor contact isdeposited as a layer and then patterned. The metal sensor contact shouldhave profile that mates to the profile of the contact via well andextends up and over the insulation layer for contact with the sensor.Often, the metal contact will have a dimple over the contact via well asthe metal is deposited evenly over the contour of the contact via well.Various processes have been used to accurately form the profile of themetal sensor contact during well filing.

The tape liftoff process applies an adhesive tape to the depositedsensor contact metal layer. The adhesive tape makes adhesive contactwith the sensor contact metal except over the contact via where thedimple is created in the surface of the metal sensor contact layer. Asthe contact metal being is deposited down into the contact via well, asurface dimple is created. As the adhesive tape is pulled away from themetal sensor contact layer, the contact layer is removed, except wherethe dimples are located. Hence, the metal sensor contact survives andremains in the contact via wells. The tape liftoff process is imprecisein forming a metal sensor contact profile and creates ragged edges andstresses in the metal contact, leading to separation failures. Liftoffpatterning processes require stepped slopes in the contact wells andconstrain the metalization layer to small thicknesses. The Liftoffprocesses are incompatible with good step coverage and depositiontechniques, such as sputtering.

The subtractive process also first deposits a metal sensor contactlayer. Patterned photoresist portions are formed over the contact wells,exposing the metal contact layer but not over the contact well. Themetal sensor contact layer is removed by a dissolving solution. Themetal sensor contact layer is dissolved save the protected metal sensorcontacts under the patterned photoresist portions. Then, the patternphotoresist portions are removed exposing the metal sensor contacts thathave upwardly extending flanges created on the side walls of the metalcontact via and lying upon the insulating layer. The problem withsubtractive process is that during the metal sensor contact layerremoval step, the metal sensor contacts are undercut under the edges ofthe pattern photoresist portions leading to imprecise metal sensorcontact profiles and flange formation. The metal sensor contacts mayalso fail to sufficiently adhere to the subsequently deposited sensor.

The chlorobenzene liftoff process uses a single photoresist layer tocreate large sized sensor contact profiles, the flanges of which can belarge. The chlorobenzene liftoff process creates a lip in thephotoresist layer that can be damaged during sputtering or heateddepositions leading to imprecise formation of the sensor contactprofiles. Chemical hazards are disadvantageously created when usingexotic and unfamiliar chemicals, such as chlorobenzene, to modify thephotoresist.

The multiple layer photoresist process uses multiple layers ofphotoresist that when respectively repeatedly applied, exposed and thendeveloped, create a thick photoresist via through which the metal sensorcontact is deposited to create a unique gold contact profile. Themultiple layer photoresist process suffers from the repeated photoresiststeps and requires very accurate process controls.

As such, conventional techniques for contact formation disadvantageouslysuffer from imprecise formations leading undesirable profiles of themetal sensor contact. Often, the metalization layer, including the metalcontact can have undesirable contours, such as the metal contactdimples. Conventional etch back methods have been used to removeundesirable surface contours of previously patterned layers. The etchback methods are used for ensuring continuous step coverage and forreflattening the surface for further high resolution photolithography.That is, the etch back method is applied to previously patterned layers.In the case of the CMOS planar etch back method, a metal contact layer,such as tungsten, is deposited over the contact well creating a dimplein the metal layer over the contact well. Because further processes mayrequire substantially flat surfaces, the dimple is removed by a planaretch back process. An insulating layer, such as glass, is reflowed byheat, onto a metal layer. Phosphosilicate glass is applied by chemicalvapor deposition and can be reflowed at high temperatures of about 700C.The reflowed glass layer is then etched back to expose the metal layerhaving surviving portions of the reflowed glass in the dimples. Next,the metal layer is etched back down to the insulation layer where thecontact well is then filled with the metal and the surface is thensubstantially flat. The tungsten layer is effectively patterned into thewells solely by the preexisting lithography. Next, the metal layer isagain deposited on a flat surface forming a metalization layer with aflat surface. The flat metalization surface can then be etched topattern the metal layer without having the contact well dimples. ThisCMOS planar etch back process provides a metal contact profile that hasno dimples. However, the CMOS planar etch back process disadvantageouslyrequired two metal deposition processes and two metal etching processes,and results in flat metal sensor flanges that may be unsuitable forconnection to MEMS sensors. These and other disadvantages are solved orreduced using the invention.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method for forming a metalcontact in a contact well.

Another object of the invention is to provide a method for forming in acontact well, a metal contact having upwardly extending metal contactflanges.

Yet another object of the invention is to provide a method for formingin a contact well, a metal contact having a metal contact dimple.

Still another object of the invention is to provide a method for formingin a contact well, a metal contact having a metal contact dimple, and anupwardly extending metal contact flange.

Still a further object of the invention is to provide a method forforming in a contact well, a metal contact having a metal contactdimple, and an upwardly extending metal contact flange for connecting toa sensor.

A further object of the invention is to provide a method for forming ina contact well, a metal contact having a metal contact dimple, and anupwardly extending metal contact flange using polymethylmethacrylate(PMMA) etch back.

Yet a further object of the invention is to provide a method for formingin a contact well, a gold contact dimple, and an upwardly extendingmetal contact flange using PMMA etch back.

The method is directed to the fabrication of metalized well contacts,such as gold well contacts, for electrical connection betweensemiconductor microcircuits and microelectromechanical systems (MEMS)devices and sensors, using standard metalization and etch processes witha minimum of subsequent photolithographic processing tools and steps.The method can be performed on variously sized substrates. The methodcan be used in a variety of fabrication processes for integrating MEMSdevices and sensors with semiconductor devices, and is particularly wellsuited for integrating chemical sensors with conventional metal oxidesilicon (MOS) semiconductor processes, such as complementary metal oxidesilicon (CMOS) processes. The method can be applied to MEMS devicesintegrated with conventional semiconductor processes, such as CMOSprocesses, that can not tolerate a heavy metal, such as a gold metalthat acts as an impurity and leads to failure of many silicon devices.

In the preferred form, a complementary metal oxide silicon (CMOS)semiconductor process device, such as a CMOS amplifier having etch runconnection under a metal contact well in an insulating layer, isconnected to an organic sensor applied to a metal sensor contact in themetal contact well. In the preferred form, the contact metal is gold forelectrochemical stability in the presence of a chemical sensor. The useof gold offers good electrical conductivity and high non-corrosiveness.The semiconductor device can be made on large diameter wafers and hencethe method offers the potential of an economy of scale when integratingsemiconductor processes with inherently incompatible MEMS devices andchemical sensors.

The method is particularly adapted to forming a metal contact with adesirable profile for secure contact with a corrosive chemical sensor.Particularly, a patterning layer, such as a photoresist (PR) layer, isdeposited for forming a larger sized patterned contact via over theinsulating layer via, for creating upwardly extending metal contactflanges. The patterned contact via effectively increases the well depthand width, while the sidewalls of the patterned contact via provide abottom surface from which the flanges upwardly extend. The metalizationcontact layer, such as a gold layer, is deposited over the patternedcontact via and insulation contact via in the insulation layer thenforming the metal contact with the upwardly extending flanges and with adimple in the metal contact layer over the contact via using a singlemetalization deposition step. After metalization, a thick planarizationlayer, such as a thick layer of PMMA is deposited for filling in thedimple. The planarization layer is then etched back exposing themetalization layer while the dimple and the upwardly extending flangesremain covered with the PPMA. The metalization layer is then removedsave the metal contact protected by the PMMA within the contact dimple.The photoresist layer and the remaining PMMA in the contact dimple arethen removed to expose the metal contact including the upwardlyextending flanges. The flanges extend upward about the height of the PRlayer. Hence, the PR layer is used to form the profile of the upwardlyextending flanges of the metal contact. With the metal flanges extendingupwardly, a chemical sensor or MEMS device can be deposited onto orconnected to the upwardly extending flanges of the metal contact. Theupwardly extending flange portion of the metal contact and the overallsize and shape of the metal contact profile can be precisely formed. Themetal contact is suitable for electrical contact between chemicalsensors and the underlying semiconductor devices using a minimum numberof process steps compatible with existing semiconductor processes. Theseand other advantages will become more apparent from the followingdetailed description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is diagram of layers after process deposition.

FIG. 1B is a diagram depicting the layers after polymethylmethacrylate(PMMA) etch back.

FIG. 1C is a diagram depicting the layers after gold pattern etch.

FIG. 1D is a diagram depicting the layers after sensor deposition.

FIG. 2 is a flow diagram of the gold contact PMMA etch back process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the invention is described with reference to thefigures using reference designations as shown in the figures. Referringto both of the figures, a preexisting substrate 10, such as a siliconsubstrate may be supporting a semiconductor device, such as a CMOSdevice, comprising a conducting portion, such as aluminum layer 12,deposited during step 30. An insulating layer 14 is deposited duringstep 32. The insulating layer 14 is preferably a glass layer made ofsilicon dioxide. During step 32, a contact via 15 is formed as apassageway through the insulating layer 10 to the semiconductor deviceconducting layer 12. A via layer 16 is deposited over the insulationlayer 14 during step 34. The via layer 16 is a patternable and removablelayer, such as a preferred patterned photoresist (PR) layer. The vialayer 16 is processed to form a flange via 17 that is larger than andcentered over the contact via 15. After applying the via layer 16 to adesired thickness, exposing the via layer 16 for patterning, anddeveloping for via layer 16 for flange via removal, the PR via layer 16includes the flange via 17 that is substantially larger than the contactvia 15 in the insulation layer 14. Then, the metalization layer 18, suchas a sputtered gold (Au) layer, is deposited during step 36 over the vialayer 16, into the flange via 17, into the contact via 15, and onto theconducting layer 12. As may be apparent, the via layer 16 serves tocreate a flange step in the dimpled portion of the metalizationconduction layer 18 over the flange via 17. Next, an etch backplanarization layer 20, such as a polymethylmethacrylate (PMMA) etchback layer 20, is deposited typically by flooding during step 38, overthe conducting layer 18 and into the stepped dimple of the conductingmetalization layer 18. The PMMA, planarization layer 20 is then etchedback during step 40, to expose the conducting layer 18, but not toexpose the stepped dimple of the conducting layer 18 then forming asurviving PMMA dimple portion 22 of the PMMA layer 20 that remains inthe stepped dimple portion 24 of the conducting layer 18. The exposedportion of the conducting layer 18 is then removed during step 42,typically by chemical etching, for exposing surviving portions of thevia layer 16, with the contact dimple portion, that is, the metalcontact 24 surviving under the surviving PMMA dimple portion 22 of thePMMA planarization layer 20. The surviving portion 22 of theplanarization layer and the PR layer 16 are removed during step 44 tofully expose the metal contact 24. As may now be apparent, the metalcontact 24 has upwardly extending flanges, shown on the left side andright side of the profile of the metal contact 24. Finally, a MEMSdevice, such as a chemical sensor 26, can be connected to or patternedonto the substrate 10 by direct deposition onto the semiconductor deviceinsulation layer 14 in electrical contact with the metal contact 24. Themetal contact 24 provide an electrical connection between the conductionlayer 12 of a semiconductor device and the MEMS sensor device 26. Anupwardly extending flange of the metal contact 24 is in electricalcontact with and can penetrate into the sensor device 26 so that themetal contact 24 makes good electrical connection between the sensordevice 26 and the semiconductor device conductor 12.

The via layer 16 has the flange via 17 having a sloped side wall thatparticularly serves to form a sloping surviving portion of theconducting layer 18 that becomes the upwardly extending flange of themetal contact 24 after removing the exposed portions of the conductinglayer 18 and the patterning via layer 16. Using PR for the via layer 16,the PR will not be under cut over the conducting layer 18. Modestproximal migration of the gold atoms about the contact 24 over theconduction layer 12 should be distal to locations where gold impuritymight cause failure in a distal semiconductor device, not shown,connected to the MEMS device 26 through the conducting layer 12. In thismanner, MEMS devices or chemical sensor devices can be integrated withsemiconductor devices on the same semiconductor substrate 10 withoutdetrimental atomic migration effects.

The PMMA etch back method can use industry standard photoresistpatterning processes on the wafer to define the pattern of thesubsequently applied metalization layer 18. The method takes advantageof high resolution photolithography, high reliability, and large waferprocesses that can be performed in large volumes to offset the cost ofexpensive equipment without conflict to process compatibility. Themethod enables arbitrary metalization or conducting etch run patterningbetween the MEMS devices 26 and the semiconductor devices. Themetalization or conducting run patterns are derived from high resolutionphotolithographic processes thereby conforming to and integrating withthe underlying small scale semiconductor devices, such as CMOSsemiconductor devices.

In the preferred form, the gold metalization layer 18 is applied overthe wafer including the substrate 10 and over the PR layer 16 coveringthe whole wafer. The metalization layer can be deposited by vacuumevaporation or sputtering. A flowable thermoplastic planarization layer20, such as the PMMA layer 20, is applied to the wafer and over themetalization layer 18. Surface tension of the thermoplasticplanarization layer 20 tends to form a nearly planar top surface overunderlying contours including the underlying contact dimples. Typically,the PMMA is applied during spin coating as a solution of thethermoplastic material in a volatile solvent. The planarization layer 20may also be applied by reflow during heating and melting of thethermoplastic material. Flowable inorganic substances may be used forthe planarization layer 20 as well. The planarization layer 20 providesa thick flat surface that can be uniformly etched and removed down tothe conduction layer 18 so as to cover the dimple portion 22 and protectthe metal contact 24 including the upwardly extending flanges from beingremoved during chemical removal of the metalization layer 18. The etchback is a planar surface removal process typically performed by etchingor lapping at a controlled rate and uniformly across the wafer, such asby oxygen plasma processing, until the underlying metalization layer 18is exposed at high points but not at low points of the underlyingcontours of the top surface of the metalization layer 18. When PR isused as the via layer 16, the photoresist tends to dominate the contourof the top surface of the conduction layer 18. The exposed portion ofthe metalization layer 18 is etched selectively with conventionaltechniques that erode completely through the metal layer 18. Typically,metalization often consists of two or three functional layers, forvarious functions including adhesion, diffusion barrier, and electricalconductivity. Sufficient metalization etching would then be needed toremove all of the functional layers of the metalization layer 18 in theexposed areas so as to expose the via layer 16 but not the metal contact24.

For isotropic etchants such as wet chemicals, some recession of themetal edge occurs as an over etch margin. The nearly vertical portionsof the metalization, covering the slopes of the resist, tend to providesome sacrificial etch distance, allowing over etching to occur withoutdegrading the lateral dimensions of the metalization pattern. The methodis well suited for wet chemical etching. The flowable plastic of theplanarization layer and the PR of the via layer should be removed byprocesses that do not attack the underlying wafer or the appliedmetalization layer 18. For example, organic solvents or oxygen plasmamay be used.

In a broad sense, the etch back method is positively used to createprofile features of a metal contact, that is, the upwardly extendingflanges of metal contact 24. The etch back method is used to define theactual shape of the metalization layer 18 disposed on the via layer 16and under the planarization layer 20. The PR is used to not only patternthe metalization layer 18 into the contact via 15, but also to formvertical aspects of the profile of the metal contact. The method isapplicable to a variety of semiconductor materials and metals, such aspolysilicon or tungsten. The use of a via layer 16, such as therelatively vulnerable photoresist, under the conducting material layer18, offers an ability to uniquely form the conducting layer 18 in thevertical dimension, in combination with a subsequent etch back layer 22,that is preferably a plastic coating planarization layer 20. In thebroad aspect, the method provides a process sequence for forming desiredthree dimensional conductive metal contact structures. In the preferredform, the contract 24 is used to electrically connect an underlyingsemiconductor device with an overlying MEMS device integrated on thesame substrate, but is applied generally for forming the profile of themetal contact 24. The method offers a standard semiconductor integrationprocess for MEMS devices or chemical sensor devices with standardizedsemiconductor devices using a variety of conventional materials. Thoseskilled in the art can make enhancements, improvements, andmodifications to the invention, and these enhancements, improvements,and modifications may nonetheless fall within the spirit and scope ofthe following claims.

What is claimed is:
 1. A method of forming a contact for connecting to acircuit having a conductor accessible through a contact via in aninsulating layer deposited on a substrate, the method comprising thesteps of, via layer depositing and patterning a via layer over theinsulating layer, the via layer having a flange via over the contactvia, conducting layer depositing a conducting layer over the via layer,the conducting layer being deposited onto the conductor through thecontact via and the flange via, the conducting layer forming a steppedcontact dimple contact via and the flange via, etch back layerdepositing an etch back layer over the conducting layer and over thecontact via and over the flange via, a top surface of the etch backlayer over the contact via and flange via is higher than a top surfaceof the conducting layer, back etching the etch back layer for exposingan exposed portion of the conducting layer for providing a survivingetch back dimple portion of the etch back layer over the contact via andthe flange via, the etch back dimple portion of the etch back layercovering a conducting dimple portion of the conduction layer, andexposed portion removing the exposed portion of the conducting layer,the conducting dimple portion becoming the contact.
 2. The method ofclaim 1 further comprising the step of dimple portion removing the etchback dimple portion for exposing the contact.
 3. The method of claim 1wherein, flange via is an aperture in the via layer, contact via is anaperture in the insulation layer, and flange via aperture is greater insize than the contact via aperture.
 4. The method of claim 1 wherein,the substrate is a silicon substrate, the insulation layer is a silicondioxide insulation layer, and the conductor is a metal.
 5. The method ofclaim 1 wherein, the via layer is photoresist.
 6. The method of claim 1wherein, the etch back layer is PMMA.
 7. The method of claim 1 wherein,the conductor layer is a gold layer.
 8. The method of claim 1 wherein,the conductor layer is polysilicon.
 9. The method of claim 1 wherein,the circuit is a CMOS circuit.
 10. The method of claim 1 wherein, theconductor layer is a composite conducting layer comprising an adhesionlayer, a barrier layer, and a conduction layer.
 11. The method of claim1 wherein, the insulation layer is glass.
 12. The method of claim 1wherein the etch back deposition step, the etch back layer is depositedby spin flooding.
 13. The method of claim 1 wherein the via layer ismade of photoresist and the via layer deposition step comprises thesteps of, applying the photoresist, mask exposing the photoresist, anddeveloping the photoresist.
 14. A method of forming a contact forconnecting a device to a circuit having a conductor accessible through acontact via in an insulating layer deposited on a substrate, the methodcomprising the steps of, via layer depositing and patterning a via layerover the insulating layer, the via layer having a flange via over thecontact via, conducting layer depositing a conducting layer over the vialayer, the conducting layer being depositing onto the conductor throughthe contact via and the flange via, the conducting layer forming astepped contact dimple contact via and the flange via, etch back layerdepositing an etch back layer over the conducting layer and over thecontact via and over the flange via, a top surface of the etch backlayer over the contact via and flange via is higher than a top surfaceof the conducting layer, back etching the etch back layer for exposingan exposed portion of the conducting layer for providing a survivingetch back dimple portion of the etch back layer over the contact via andthe flange via, the etch back dimple portion of the etch back layercovering a conducting dimple portion of the conduction layer, exposedportion removing the exposed portion of the conducting layer, theconducting dimple portion becoming the contact having an upwardlyextending flange upwardly extending from the insulation layer, dimpleportion removing the etch back dimple portion, and device depositing thedevice onto the upwardly extending flange.
 15. The method of claim 14wherein, the device is a chemical sensor.
 16. The method of claim 14wherein, the via layer is photoresist, the etch back layer is PMMA, theconductor layer is a gold layer, the circuit is a CMOS circuit, and thedevice is a MEMS device.
 17. The method of claim 14 wherein, the vialayer is photoresist, the etch back layer is PMMA, the conductor layeris a gold layer, the circuit is a CMOS circuit, and the device is achemical sensor.
 18. The method of claim 14 wherein, the via layer isphotoresist, the etch back layer is PMMA, the conductor layer is a goldlayer, the circuit is a CMOS circuit, and the device is a biochemicalsensor.
 19. The method of claim 14 wherein, the via layer isphotoresist, the etch back layer is PMMA, the conductor layer is a goldlayer, the circuit is a CMOS circuit, the device is a chemical sensor,flange via is an aperture in the via layer, contact via is an aperturein the insulation layer, flange via aperture is greater in size than thecontact via aperture, the substrate is a silicon substrate, theinsulation layer is a silicon dioxide insulation layer, and theconductor is a metal.